Process for the recovery of krypton and/or xenon by low-temperature separation of air

ABSTRACT

The process is used for the recovery of krypton and/or xenon by low-temperature separation of air. Compressed and purified feed air ( 1, 1 A,  1 B,  29 ) is introduced into a rectification system for nitrogen-oxygen separation, which has a high-pressure column ( 2 ) and a low-pressure column ( 3 ). An oxygen-rich liquid ( 42 ) is removed at a first point from the low-pressure column ( 3 ). At least a first portion ( 43 ) of the oxygen-rich liquid ( 42 ) is re-introduced in the low-pressure column at a second point, whereby a mass exchange section ( 44 ) is arranged between the first and second points on the low-pressure column. An additional liquid ( 35 ) is drawn off from a third point of the low-pressure column ( 3 ), which is arranged at the level of the second point or below it, said liquid introduced into the evaporation chamber of a first condenser-evaporator ( 4 ) and partially evaporated there. A krypton- and xenon-containing fraction ( 138 ) is drawn off from the evaporation chamber of a first condenser-evaporator and introduced into the evaporation chamber of a second condenser-evaporator ( 27 ). A krypton-xenon concentrate ( 125 ) is drawn off from the second condenser-evaporator ( 27 ).

The invention relates to a process according to the introductory clause of claim 1, which is used for the recovery of krypton and/or xenon by low-temperature separation of air.

The principles of low-temperature separation of air in general as well as the design of the rectification system for nitrogen-oxygen separation in particular are described in the treatise “Tieftemperaturtechnik [Low-Temperature Technology]” by Hausen/Linde (2^(nd) Edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). The high-pressure column is operated under a higher pressure than the low-pressure column; the two columns preferably are in a heat-exchange relationship with one another, for example via a main condenser, in which top gas from the high-pressure column is liquefied against evaporating bottom liquid from the low-pressure column. The rectification system of the invention can be designed as a standard double-column system but also as a three-column or multiple-column system. In addition to the columns for nitrogen-oxygen separation, additional devices for recovering other air components, in particular noble gases, can have, for example, an argon recovery component.

The krypton-xenon concentrate that is produced can be separated into pure krypton and/or xenon on the spot in additional process steps. As an alternative, it is stored, for example, in a liquid tank, and further processed at another site.

A process of the type mentioned above is described in U.S. Pat. No. 5,313,802.

An object of the invention is to execute such a process in an especially economical way.

At least this object is achieved by the features as described herein.

Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.

According to the invention, the krypton- and xenon-containing fraction from the evaporation chamber of the first condenser-evaporator is introduced into the evaporation chamber of a second condenser-evaporator and partially evaporated there; the remaining liquid portion is drawn off from the second condenser-evaporator as a krypton-xenon concentrate.

In this way, a separate crude krypton-xenon column is not absolutely required. In the interaction of the second condenser-evaporator and the mass exchange section between the first and second points on the low-pressure column, methane can nevertheless escape from the lower area of the low-pressure column without occurring in excessively high concentrations in the krypton-xenon concentrate.

The “mass exchange section” can be formed by the plate, filling materials and/or orderly packing. It has, for example, a range of 1 to 10, preferably 2 to 6, theoretical or actual plates. (For the case where only actual plates are used as mass exchange elements in the section in question, the figures pertain to the numbers of actual plates; if packing, filling materials or combinations of various types of mass exchange elements are used, the figures can be applied to the number of theoretical plates.)

The second condenser-evaporator is preferably arranged as a secondary condenser outside of the low-pressure column and is designed as a bath evaporator.

The first condenser-evaporator can be arranged inside or outside of the low-pressure column. The mass exchange section between the first and second points on the low-pressure column can be arranged in the same container as the other sections of the low-pressure column or in a separate container.

The second condenser-evaporator is heated by a condensing gas preferably being directed through its liquefaction chamber. In this case, it can be, for example, a gas that is approximately below the operating pressure of the high-pressure column, in particular a partial stream of the feed air.

The methane that rises in the mass exchange section between the second and first points on the low-pressure column preferably leaves the low-pressure column with an oxygen product stream. The latter can be drawn off either in liquid form, for example as a second portion of the oxygen-rich liquid from the first point on the low-pressure column, or as a gas stream from the same point or a higher point of the low-pressure column.

In principle, the first condenser-evaporator, which usually represents the main condenser of the two-column system, can be designed in any known form, for example as a falling-film evaporator. In the invention, it is more advantageous, however, to create it as a bath evaporator, since in this case, a relatively high concentration of krypton and xenon can already be achieved there in a reliable way.

In addition to or as an alternative to the liquid from the evaporation chamber of the first condenser-evaporator, another krypton- and xenon-containing stream can be drawn off from the high-pressure column and directed into the evaporation chamber of the second condenser-evaporator.

In this way, a large proportion of the krypton and xenon that are contained in the feed gas passes directly into the second condenser-evaporator, without first flowing through the low-pressure column. Thus, less krypton and xenon with the products of the low-pressure column are lost. Also, other volatile substances that are heavier than oxygen, such as hydrocarbon or N₂O, do not pass to the bottom of the low-pressure column and to the first condenser-evaporator.

It is advantageous when the additional krypton- and xenon-containing stream is directed upstream from its introduction into the evaporation chamber of the second condenser-evaporator through a purification stage, which is designed in particular as a stripping column, into whose top the additional krypton- and xenon-containing stream is released in liquid form. One such additional column is described in detail in DE 10332863 A1.

Preferably at least a portion of the gas, which is formed in the evaporation chamber of the second condenser-evaporator, is used as rising steam from the stripping column.

The additional krypton- and xenon-containing stream can be partially evaporated upstream from its introduction into the evaporation chamber of the second condenser-evaporator of a purification stage or upstream from the purification stage in a third condenser-evaporator, in particular the top condenser of a crude argon column. The portion that remains liquid is then fed to the purification stage or to the second condenser-evaporator.

The invention as well as further details of the invention are explained in more detail below based on the embodiments depicted in the drawings. In this connection:

FIG. 1 shows a first embodiment of a process according to the invention without recovery of argon, and

FIG. 2 shows a first embodiment of a process according to the invention with argon recovery.

Compressed and purified feed air (1) is introduced into a first portion 1A in a rectification system for nitrogen-oxygen separation, which has a high-pressure column (2), a low-pressure column (3), and a main condenser (“first condenser-evaporator”) 4, which is designed in the example as a falling-film evaporator. Gaseous nitrogen 5 from the top of the high-pressure column is fed to a first portion 6 in the condensation chamber of the main condenser 4. The condensate 7 that is formed there is released to a first portion 8 of the high-pressure column as a reflux. A second portion 9 is fed to its top via a butterfly valve 10 in the low-pressure column 3. Another portion 11 of the gaseous nitrogen 5 from the top of the high-pressure column is fed into a circuit, not shown, liquefied there and released to the high-pressure column 2 via line 12 as an additional reflux.

An oxygen-enriched liquid 40, which is essentially free of heavily volatile components, is drawn off approximately 3 to 5 theoretical or actual plates above the bottom of the high-pressure column 2 and fed to the low-pressure column at an intermediate point via a butterfly valve 41.

From the low-pressure column 3, gaseous nitrogen 32 is drawn off at the top as product (GAN) or residual gas. An oxygen-rich liquid 42 is removed from a first point on the low-pressure column and is fed again to a first portion 43 of the low-pressure column 3 at a second point, which is below a mass exchange section 44. The bypass amount 43 and thus the reflux ratio in the mass exchange section 44 is set via a bypass valve 46.

A second portion 45 of the oxygen-rich liquid 42 is subjected to an internal seal (internal compression) by being brought into a pump 47 to the desired product pressure and being fed via line 48 (LOX-IC) to one or more heat exchangers, for example the main heat exchanger system, in which the feed air 1 is also cooled. There, it is evaporated (or, in the case of supercritical product pressure—pseudo-evaporated), heated to approximately ambient temperature and finally drawn off as a gaseous pressure product.

Evaporation and heating can be performed in, for example, indirect heat exchange with a high-pressure air stream. In the embodiment, however, the nitrogen circuit that is indicated in the lines 11 and 12 is used for this purpose.

Below the mass exchange section 44, another liquid 35 is drawn off, conveyed by means of another pump 36 via line 37 to the evaporation chamber of the main condenser 4 and partially evaporated there. The steam-liquid mixture 38 that is formed in this case flows back partially to the bottom of the low-pressure column 3. Another portion that remains liquid is directed via line 138 into the evaporation chamber of a second condenser-evaporator, the secondary condenser 27. The fraction that remains liquid after the partial evaporation in the secondary condenser is removed as a krypton-xenon concentrate 125 from the evaporation chamber of the secondary condenser 27 (Crude Kr—Xe).

The secondary condenser 27 can be heated with any suitable fraction. In the embodiment, a portion 1B of the cold feed air 1 is used as a heating agent. (As an alternative to this, any other fraction from the high-pressure column, for example, compressed nitrogen from the top of the high-pressure column 2, could be used.) The liquefied air 29 in the secondary condenser 27 is introduced into the high-pressure column 2 several plates above the gaseous air 1.

Oxygen-enriched bottom liquid 13 is removed as an “additional krypton- and xenon-containing stream” from the high-pressure column 2, depressurized in valve 14 and released to the top of an additional column (“stripping column”) 120, which is operated as a “purification stage” at about the same pressure as the low-pressure column 3. The purified additional krypton- and xenon-containing stream 121 is fed as an additional or single feedstock to the evaporation chamber of the secondary condenser 27.

Gas 30 that is formed in the evaporation chamber of the secondary condenser 27 is introduced into the stripping column 120 at least partly as rising steam 31. A nitrogen-containing gas 165 is drawn off from the top of the stripping column 120 and fed to a suitable point on the low-pressure column 3 via line 166.

The embodiment of FIG. 2 has an argon recovery component, in contrast to FIG. 1.

An argon-containing fraction from the low-pressure column 3 is directed into a crude argon column 19 via an argon transfer line 48. The argon-containing fraction 48 is fed in gaseous form to the crude argon column 19 right at the bottom. The rising steam accumulates in the argon.

At the top of the crude argon column 19, argon-enriched steam 50 is produced and is condensed to a large extent in a “third condenser-evaporator,” the crude argon condenser 17. The liquid 51 that is produced in this case is released as reflux liquid to the crude argon column 19. Bottom liquid 55 of the crude argon column 19 flows back into the low-pressure column 3.

Crude argon 58 that remains in gas form from the liquefaction chamber of the first condenser-evaporator 17 is optionally further separated in a pure argon column, and in particular more volatile components, such as nitrogen, are removed from it (not shown).

The oxygen-enriched bottom liquid 13 is fed here first as coolant to the evaporation chamber of the crude argon condenser 17 via a valve 16. Only the portion that remains liquid 26 is released to the stripping column 120. The steam 25 that is produced in the partial evaporation in the third condenser-evaporator 17 is drawn off from the evaporation chamber of the first condenser-evaporator 17 and fed together with the top steam of the stripping column to the low-pressure column via line 165.

The entire disclosure of all applications, patents and publications, cited herein and of corresponding German application No. 102005040508.8 filed Aug. 26, 2005 is incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for the recovery of krypton and/or xenon by low-temperature separation of air, which comprises: introducing compressed and purified feed air (1, 1A, 1B, 29) into a rectification system for nitrogen-oxygen separation, which has at least a high-pressure column (2) and a low-pressure column (3), removing an oxygen-rich liquid (42) at a first point from the low-pressure column (3), re-introducing at least a first portion (43) of the oxygen-rich liquid (42) in the low-pressure column at a second point, whereby a mass exchange section (44) is arranged between the first and second points on the low-pressure column, drawing off a liquid (35) from a third point of the low-pressure column (3), which is arranged at the level of the second point or below it, and introducing it into the evaporation chamber of a first condenser-evaporator (4) and partially evaporating it there, and drawing off a krypton- and xenon-containing fraction (138) from the evaporation chamber of the first condenser-evaporator (4), introducing the krypton- and xenon-containing fraction (138) into the evaporation chamber of a second condenser-evaporator (27), and drawing off a krypton-xenon concentrate (125) from the second condenser-evaporator (27).
 2. A process according to claim 1, wherein the second condenser-evaporator (27) is heated with a gas (1B) that has a pressure that is approximately equal to the operating pressure of the high-pressure column.
 3. A process according to claim 1, wherein the second condenser-evaporator is heated with a partial stream (1B) of the charging air (1).
 4. A process according to claim 1, wherein a second portion (45, 48) of the oxygen-rich liquid (42) from the first point on the low-pressure column (3) and/or another oxygen fraction from above the first point of the low-pressure column is removed as product.
 5. A process according to claim 1, wherein the first condenser-evaporator (4) is designed as a bath evaporator.
 6. A process according to claim 1, wherein another krypton- and xenon-containing stream (13) is drawn off from the high-pressure column (3) and is introduced (121) into the evaporation chamber of the second condenser-evaporator (27).
 7. A process according to claim 6, wherein the additional krypton- and xenon-containing stream (13, 26) is directed upstream from its introduction into the evaporation chamber of the second condenser-evaporator (27) through its purification stage.
 8. A process according to claim 7, wherein the purification stage is designed as a stripping column (120).
 9. A process according to claim 8, wherein gas (30, 31) that is formed in the evaporation chamber of the second condenser-evaporator (27) is introduced into the stripping column (120) at least partly as rising steam.
 10. A process according to claim 6, wherein the additional krypton- and xenon-containing stream (13) is partially evaporated upstream from its introduction into the evaporation chamber of the second condenser-evaporator.
 11. A process according to claim 8, wherein the additional krypton- and xenon-containing stream (13) is partially evaporated upstream from the purification stage (120) in a third condenser-evaporator (17).
 12. A process according to claim 11, wherein the third condenser-evaporator (17) is a top condenser of a crude argon column (19). 